Electrical capacitors made of thin film dielectric material which is metallized on both sides are used in large quantities in electrical devices because of their reliability, their desirable electrical characteristics and their relatively low cost. U.S. Pat. No. 2,470,826 issued on May 24, 1949 to W. McMahon illustrates a capacitor in which a double sided metallized dielectric layer is folded an odd number of times in a longitudinal direction. Another double sided pleated metallized dielectric layer capacitor is disclosed in U.S. Pat. No. 3,854,075 issued Dec. 10, 1974, to John Phillip Uhl, and assigned to the assignee of the present invention. In the Uhl patent, a capacitor is disclosed which is formed by pleating a double side metallized dielectric layer an even number of times and then winding the structure about the leads which act as mandrels and are left in the structure. Spraying of the ends of the wound capacitor structure is, therefore, not needed or contemplated by the Uhl patent. The McMahon patent suggests that his dielectric material may then be wound along its longitudinal dimension after pleating and that end terminations may then be applied to a wound capacitor segment by spraying metal against the edges of the wound capacitor segment. The types of capacitors envisioned by the McMahon and Uhl patents, if commercially practicable, would be extremely desirable because substantially all of the dielectric layers of these capacitors are in the electric field and these capacitors may be made without the insertion of an additional dielectric layer in the main body of the capacitor, although a short initial unmetallized dielectric strip and a protective terminating dielectric strip could be employed with these capacitors, if desired.
The types of capacitor construction that were suggested in the McMahon and Uhl patents have a number of decided manufacturing and electrical advantages. For example, the problem of masking the dielectric layer during electrode evaporation when the electrode areas are applied to the film is substantially reduced. In addition, no slitting is required as it is during the manufacture of conventional wound film capacitors. Furthermore, the capacitance per unit dielectric area is maximized because of the substantially full use of the dielectric layer in the electric field. Shorter winding lengths per unit capacitance are thereby required, reducing the winding labor. The dielectric lengths can also be calibrated and pre-cut to produce a capacitor of more accurate values. Another advantage of these types of capacitors, if they can be made in a reliable manner, is that there are no air layers between the metallized electrodes and the pleated film surfaces and this substantially increases the voltage, for example, one hundred volts or more, at which corona discharge starts, making these capacitors commerically attractive for new applications such as fluorescent lighting circuits.
However, the construction of commercially successful capacitors as suggested in the McMahon and Uhl patents was hampered by problems that are presented by the attachment of leads to the wound capacitor segments. The McMahon patent suggested that leads could be attached to his capacitor by the well-known Schoop process. The application of high velocity metallic spray coatings to the edges of the capacitor described by McMahon is not commercially practical, however, because the spray will penetrate through the dielectric area at a pleat and thereby short one electrode to the other. If the temperature and pressure of the spray are lowered, penetration of the dielectric layers may be reduced but the adhesion of the metallic spray will generally be very poor. The multiple leads suggested in the McMahon patent also are not satisfactory since they tend to tear the dielectric material and to make the capacitor bulky and inductive.
In order to solve the above mentioned spray shorting problem at a pleated edge, it has been suggested in Australian Patent No. 159,958, patented on Nov. 24, 1954 to provide unmetallized dielectric strips behind all of the pleats of a pleated metallized capacitor. While this solution may provide a technically feasible method of attaching leads to a pleated metallized dielectric layer capacitor, the number of unmetallized strips that are required on the dielectric layer greatly complicates the manufacturing process for both the dielectric layer itself and the pleated capacitor product. For example, in a capacitor having five pleats, there must be, in addition to the two unmetallized margin areas, five unmetallized strips that run along the entire length of the dielectric material, with three of these strips being on one side of the dielectric layer and two of these strips being on the opposite side of the dielectric layer. It is apparent that the masking problems and the cost of manufacturing such a specialized item make the solution of the Australian patent commercially unattractive. This is especially true since the type of metallized dielectric layer that is used in the capacitor of the present invention, (in which only the margin areas are required), is readily commercially available.
The leads of the capacitor of the previously mentioned Uhl patent were used as mandrels to wind the pleated capacitor segment about the center, in a manner similar to that suggested in U.S. Pat. No. 2,887,649 issued May 19, 1959 to Daniel B. Peck. In the Uhl patent, conductive foil tabs were inserted between the leads and the metallized electrode areas to make positive contact with the electrode and to protect the electrode from damage by the leads during winding. However, it has been determined that this construction leads to a higher failure rate than desirable due to opening of the electrode area around the periphery of the metal foil, for reasons which are not completely understood.
The use of the leads of a wound capacitor as the winding mandrels is desirable in order to obtain secure lead attachment without the necessity of applying a metal end spray to the edges of the wound metallized capacitor segment after it is formed. U.S. Pat. No. 3,754,311 issued Aug. 28, 1973 in the name of Charles C. Rayburn, and assigned to the assignee of the present invention describes a wound film capacitor which uses the lead wires as winding mandrels.
In the capacitor of this patent a dielectric is sandwiched between two layers of metallized, or preferably foil, electrodes. A pair of lead wires are then positioned against the outer surfaces of the foils and rotated together in the manner of mandrels to wind the film and foil layers into a capacitor body. Flat, or otherwise deformed portions formed on a short portion of one end of each lead wire which is outside of the capacitor during winding are then drawn into the center of the capacitor by pulling on the other end of the wires. The capacitor body is then heated to shrink the dielectric and thereby cause the leads to be held in extremely firm engagement with the foil layers. Since the foils encircle at least 270.degree. of the periphery of the lead wires, a very firm, low resistance, pressure bond is made which renders the capacitor able to withstand substantial amounts of heat applied during a soldering operation. The elimination of mandrel holes allows the capacitor to be extremely compact and usable for many applications without the addition of additional thicknesses of sealing materials.
The capacitor of the present invention utilizes a technique similar to that of Rayburn U.S. Pat. No. 3,754,311 to achieve the winding of an initial portion of the finished capacitor of the present invention; and thereafter, a pleated, double-sided, metallized dielectric layer is incorporated into the winding process. The finished capacitor, therefore, incorporates the many advantages of both the capacitor of U.S. Pat. No. 3,754,311 and of pleated, wound capacitors.
Various deficiencies of the prior art were overcome by the capacitor of the Rayburn U.S. Pat. No. 3,754,311 which consisted broadly of two or more layers of shrinkable thermoplastic dielectric film positioned, in sandwiched relation, between a pair of foil layers or strips, the outside surfaces of the foil layers being engaged by a pair of axially extending lead wires. The lead wires have flats or other deformations thereon which lock them to the foils after winding and heat shrinking the dielectric and prevent their being rotated in or being pulled out of the capacitor. Since it has been found that the foil and dielectric strips tend to wrinkle when wound about a lead of non-uniform cross-section, the capacitor was wound on smooth portions of the lead wires and the deformed lead portions were pulled axially inside the capacitor after the winding has been finished and the wraps sealed together, but prior to the heat shrinking operation.
The capacitor of this patent utilized the heat shrinkage characteristics of bi-axially oriented films. Such films include polyester, polypropylene, polystyrene, polycarbonate and combinations thereof. A variation in particular performance characteristics such as the dissipation factor and the temperature-capacitance relation, may be obtained by using a particular film or combination of films having the characteristics desired.
This capacitor had the advantage that relatively large, rigid leads are used, e.g., No. 20 or No. 22 AWG. The leads could be formed in either axial, radial or stand-up styles.
Although the capacitor system of the Rayburn patent and the capacitor of the present invention were designed for self-encasement, a variety of other coating systems are usable as permitted within the style and temperature limitations of the materials used for this type of capacitor. For example, the units may be epoxy dipped, phenolic dipped and waxed, molded, wrap and filled, potted or canned.
Since the lead wires serve as the winding mandrel and remain in place after winding, there is no possiblity of this type of capacitor gradually unwinding into the mandrel hole as is often possible in conventional winding systems using a retractable mandrel. The solid core of the capacitor produced by the lead wires is thus a basis for stable performance characteristics. Since the outer folds which contain the margin areas extend to approximately the center of the strip, conductivity exists between each turn of the capacitor so that no end spray operation is required and the turns of the capacitor are charged in parallel rather than in series.
Unlike conventional heat shrunk tab type construction, such as exemplified by U.S. Pat. No. 2,735,970, the lead wires have pressure contact over the entire foil width and for approximately 270.degree. around their peripheries. This large amount of intimate pressure contact not only assures low contact resistance, but also permits the foils and lead wires to conduct and dissipate heat within the capacitor section and thus protect the dielectric from overheating in localized high temperature areas as the capacitor is soldered into its circuit application. By the provision of deformations such as flatted regions on the leads centrally of the capacitor section, the leads are strongly locked against translation and rotation.